1. Field of the Invention
The present invention relates to a projection lens and more particularly a projection lens for a video projector which projects an image of a CRT (cathde ray tube) on a wide screen.
2. Description of Prior Art
In a common video projector the blue(B), green(G) and red(R) images of a CRT are projected on a screen through the three corresponding lenses. Since the widths of emission spectrum of the three colors are relatively narrow, there is generally no need of an achromatic lens. Various lenses for video projectors are known and used. One of them is of the type which is composed of ordinary spherical lenses only. Another is of the type which includes one or more non-spherical surfaces. With the first-mentioned structure composed of spherical lenses only it is difficult to obtain a bright and wide angle of view. Therefore, the use of it is limited. In order to minimize the number of element lenses while maintaining the desired high performance it is essential for the projector lens to use non-spherical surfaces therein.
Some of the known projection lenses designed employing non-spherical lenses are disclosed in Japanese Patent Application laid open Nos. 124,114/1980, 108,818/1982, 118,616/1983 and 125,007/1983. All of these known projection lenses are composed of three element lenses. As an example, FIG. 1 shows the projection lens disclosed in the above-referred Japanese Patent Application laid open No. 108,818/1982 the assignee of which is the same as that of the present application.
As shown in FIG. 1, the prior art projection lens is composed of three lenses, namely a first lens of positive refractive power, a second biconvex lens of positive refractive power and a third lens of negative refractive power having a highly curved surface facing the screen side. In FIG. 1, the screen is on the left-hand side of the drawing. The original numbers of the respective lenses are from the screen side.
Comparing with the projection lens shown in FIG. 1, the construction of the projection lens disclosed in Japanese Patent Application laid open No. 124,114/1980 is different in that the refractive power of the first lens is weak and that of the second one is strong. Because of this basic form, it has a drawback that the performance of image formation becomes worse at marginal positions although the image-forming performance on axis is good. Another disadvantage of this projection lens is that the focal point varies markedly with the change in temperature of the atmosphere because the three element lenses are all made of plastics. The projection lens as shown in FIG. 1 has also some disavantages. This prior art projection lens has been designed on the assumption that the screen on which an image of a flat CRT is to be projected is a screen curved with the radius of curvature of 2,500 mm and its concave surface facing to the lens side. Therefore, this prior art projection lens has an advantage in view of field curvature. But, the angle of view obtainable by it is narrow, the half view-angle being only 24.05.degree.. Furthermore, in respect of the value of MTF(modulation transfer function) on the CRT side (see FIG. 2), the prior art projection lens is not satisfactory. The value of MTF is about 50% for 5 lines/mm. In addition, also in this projection lens, the three element lenses are all made of plastics which is greatly affected by the change of temperature of the atmosphere as noted above.
The lenses of the prior art projection lens are made of acrylic material. The refractive index n.sub.e of the acrylic lens material to the e-ray varies with changing of temperature as follows: EQU n.sub.e (20.degree. C.)=1.49380 EQU n.sub.e (0.degree. C.)=1.49608 EQU n.sub.e (40.degree. C.)=1.49134
In the above measurement, the state at 20.degree. C. is considered as the steady state of the material. As the refractive index varies with change of temperature as shown above, the focal point of the projection lens also changes with change of temperature on the side of the CRT. In the case of the projection lens shown in FIG. 1, the magnitude of defocus caused by the variation of refractive index with change of temperature is found from the above data to be as follows (the defocus is zero at 20.degree. C.):
0.degree. C.: -0.635 mm PA1 40.degree. C.: +0.749 mm
This variation of focal point is not small. Judging from the value of MTF shown in FIG. 2 it may be said that the variation is over the practically allowable range if the video projector is located at a place where the temperature changes greatly.
The projector lenses disclosed in the aforementioned Japanese Patent Application laid open Nos. 118,616/1983 and 125,007/1983 are of the same construction as that shown in FIG. 1. But, in these prior art projector lenses, the second lens is made of glass to reduce the variation of focal point with the change of temperature. However, they have still some disadvantages. Since the second lens is made of glass and is not non-spherical, the spherical aberration is not corrected well. The angle of view is narrow. The half view-angle is smaller than 25.degree.. The field curvature is unsatifactorily corrected. Even in respect of focus variation by the change of temperature, that in the first lens remains uncompensated at all. As a whole, therefore, the effect on the reduction of focal variation is not satisfactory.